GH cells are clonal strains of rat pituitary tumor cells which secrete prolactin and growth hormone and which contain specific functional receptors for somatostatin and thyrotropin-releasing hormone (TRH). The overall objectives of this project are to examine the mechanism by which the steady state levels of somatostatin and other peptide receptors are regulated in GH cells. We proposed to 1. characterize the agents which modulate the steady state concentration of somatostatin receptors available at the cell surface, 2. elucidate the nature and sequence of events involved in the processing of the somatostatin-receptor complex after initial binding of ligand and 3. determine how these processing events can be modified by agents which modulate hormone receptor levels in order to elucidate the mechanisms by which these agents act. GH cells provide a uniquely useful system to study the mechanisms and the biological consequences of peptide hormone receptor modulation because they consist of a homogeneous cell population, because they contain specific functional receptors for several peptide hormones, and because the receptors for these hormones are subject to modulation both by homologous and heterologous ligands.